The present invention generally relates to data processing systems and more particularly to input/output control associated with real time devices operating on a general purpose data processing system.
At least two types of input/output operations are known in the prior art. In a first type, an I/O program is executed within a computer going through the steps of addressing an I/O device, sending a command to the addressed I/O device to instruct the device to either send or receive data, and either receiving or sending several bytes of data to the I/O device. Usually, the amount of data transferred during one I/O device selection is kept small so that the channel will be available without long delays for receiving interrupt requests from other transfer devices requiring service. During program input/output data transfer, the data passes through the Central Processing Unit (CPU) and is stored in memory by the CPU instruction. Real time devices often use the programmed input/output data transfer method in order that the computer may have immediate access to the data being received. Immediate data access allows immediate response calculation for transmission to the real time device.
Nonreal time devices such as disk memories and tape drives can be more efficiently operated in what is sometimes termed "device initiated burst mode". Device initiated data transfer is initiated by a signal such as cycle steal request being sent from a device to a computer which terminates instruction execution after completion of the currently executing instruction. Upon termination of instruction execution, the computer sends a cycle steal grant response to all I/O devices which are connected in a daisy chain sequence. If two devices require service simultaneously, both will raise a cycle steal request signal but the device with highest priority will receive the cycle steal grant signal first and can inhibit propagation to the lower priority device. Having received the cycle steal grant signal, the disk control logic can place an identifying port address on the data bus, a command indicating whether the device which is to send or receive data and thereafter a single sequential burst of data bytes without intervening address and command information. When in cycle steal mode, data does not pass through the CPU itself, but rather is passed directly to sequential memory addresses under control of an indexing address counter in the selected port of the input/output channel. In addition to the address counter, each port has a byte counter in the associated burst mode device containing the length of the data transfer. As each byte is transferred, the count is decremented. When the count reaches zero, the data transfer has been completed and the burst mode device generates an end of message signal. Each port address counter and byte counter are loaded by programmed instructions prior to the start of any burst data transfer. If the input/output channel is slower than memory, time slots may become available within which the computer can be given access to the memory without interfering with data being transferred by the channel.
It should be noted however, that in spite of the fact that the Central Processor Unit may operate in memory, the Central Processor Unit cannot execute a programmed I/O operation to serve a real time device until the entire burst of data has been transmitted. The burst of data may constitute a single block of 256 or 1,024 bytes requiring significant transmission time and leading to excessive response time for the real time device.
Breaking up the burst into a series of short bursts may allow adequate service to the real time device but seriously impacts burst mode efficiency since the cycle steal request-cycle steal grant sequence must be repeated far more often with small bursts.
A partial solution to this problem is proposed in the prior art by permitting a higher priority device capable of device initiated data transfer to suspend transfer by a lower priority device in the middle of a burst and substitute its own port address, command and/or data sequence. The lower priority device resumes the transmission of its burst upon completion of transmission by the higher priority device. The above described partial solution is incomplete because although a real time device can provide its data into memory, it has no way of receiving a response until all of the lower priority transmitting and suspended bursts have been completed. Until completed, the channel interface is not available for programmed input/output operations by the CPU. Furthermore, the CPU may be unaware of the existence of the real time data in memory since programmed I/O interruptions are inhibited during burst mode channel operations.